The invention relates to an optical parametric oscillator (OPO) system with improved conversion efficiency and, more particularly, to an OPO system using self-seeding to improve conversion efficiency.
Optical parametric amplification (OPA) is a nonlinear optical process whereby light at one wavelength, the pump wavelength, is used to generate light at two other (longer) wavelengths in a nonlinear optical material with a nonvanishing second order nonlinear susceptibility. Optical gain is established at two wavelengths, conventionally referred to as the signal and idler wavelengths. The sum of the energies of a signal photon and an idler photon are equal to the energy of a pump photon. There is no fundamental physical distinction between the idler wave and the signal wave. An optical parametric oscillator (OPO) is a resonant optical cavity containing a nonlinear material which provides OPA when pumped by a beam of laser radiation at a pump frequency from a pump source.
The content and orientation of the crystal and the design of the resonant cavity determines the signal and idler frequencies. The gain within the nonlinear medium combined with feedback within the resonant cavity permits oscillation, a process similar to build-up in a laser cavity. The cavity can either be singly resonant in which end mirrors reflect only the signal frequency or doubly resonant in which end mirrors reflect both signal and idler frequencies. End mirrors of the OPO are often transparent to the pump frequency, although they reflect the pump in some designs. OPOs with singly resonant cavities are typically more stable in their output than OPOs with doubly resonant cavities.
A schematic diagram of a prior art 4-mirror, ring OPO appears in FIG. 1 (U.S. patent application Ser. No. 10/217,853 filed Aug. 12, 2002). FIG. 1 illustrates one embodiment of the 4-mirror, nonplanar ring system of the present invention. The system has 4 mirrors, designated as M1, M2, M3 and M4. In general, the pump laser beam enters through a partially transmissive mirror (for example, M1) and exits through another partially transmissive mirror (for example, M2) although the other mirrors could also be used to admit and emit the pump light. Mirrors M3 and M4 are identical in order to maintain polarization with only one wave plate (WP1), a half-wave plate, situated between mirrors M2 and M3, although the wave plate could also be situated between mirrors M1 and M4. This half-wave plate is included to maintain linear polarization at the crystal. These mirrors reflect the signal wave and could also reflect the idler and pump, although this is usually undesirable because it makes signal/idler wavelength selection difficult. Situated between mirrors M1 and M2 is at least one nonlinear optical medium C1 (generally a crystal).
To obtain a useful device, it is necessary to be able to choose a specific signal wavelength. This is made possible within the nonlinear material itself, as useful gain appears only when the pump wave, the signal wave, and the idler wave can propagate and stay in phase with each other. This phase matching condition is difficult to establish. Optical materials generally exhibit a property called dispersion, in which the refractive index varies with wavelength. Normally, shorter wavelength light propagates more slowly than do longer wavelengths. Consequently, as waves with different frequencies propagate they rapidly move in and out of phase with each other. The resulting interference prevents the signal wave from experiencing significant optical gain. The most common ways of phase matching are to take advantage of birefrigence often present in nonlinear crystals or to quasi-phase match by periodically changing the orientation of the nonlinear crystal to periodically rephase the pump, signal, and idler waves.
An optical parametric oscillator system that provides an improved beam is described by Nabors et al. (U.S. Pat. No. 5,781,571, issued on Jul. 14, 1998), utilizing an elongated resonant cavity with an output coupling device at one end and a Porro prism at the opposite end. Ansteft et al. (G. Ansteft, G. Goritz, D. Kabs, R. Urschel, R. Wallenstein, and A. Borsutzky, 2001, Appl. Phys. B., DOI 10.1007) describe a method for reducing beam divergence using collinear type-II phase matching and back reflection of the pump beam. Alford et al. (U.S. Pat. No. 6,147,793, issued on Nov. 14, 2000) also describe a class of optical parametric oscillators that introduce means for reducing signal losses due to backconversion of signal photons in the nonlinear optical medium. Elimination of backconversion results in improved beam quality compared with an OPO in which backconversion is present.
Another way to communicate phase across the beam is by spatial walk off between the signal and idler beams, combined with image rotations (Smith, A. and Bowers, M., presented at University of Kaiserslautern, Kaiserslautern, Germany, May 5, 2000; incorporated herein by reference). Walk off, which describes the angle difference p between the signal and idler Poynting vectors in the crystal (nonlinear medium), tends to smooth the phase of the signal beam over regions that interact with a particular portion of the idler beam. For a single pass through the crystal, this is a stripe of length equal to the walk off displacement within the crystal. Over successive passes of an OPO cavity, the stripe lengthens by this amount on each pass. This leads to a set of stripes of uniform phase oriented parallel to the walk off direction but with an independent phase for each stripe.
Nanosecond OPOs are typically pumped by laser pulses of 1–100 ns duration. OPOs driven by such nanosecond-duration pump pulses operate in a transient regime. In contrast with continuous-wave OPOs, these nanosecond OPOs never reach a steady state of oscillation. The build-up time is typically a significant fraction of the pump duration before the pump pulse is depleted, reducing conversion efficiency. This lag can be reduced by pumping the OPO with a more powerful pulse but this tends to increase the back conversion with a single pass of the OPO crystal. This back conversion converts the signal and idler energy back into pump energy, limiting the conversion efficiency. This effect is often manifested by a roll-over in the plot of signal output energy versus pump energy. Back conversion also tends to deplete the center of the signal and idler beams, reducing the quality of the beams, often quantified with M2.